The driving wheels such as rear wheels of a rear wheel drive vehicle, front wheels of a front wheel drive vehicle and all wheels of a 4WD (4-wheel Drive) vehicle are supported by a suspension system via a bearing apparatus for a driving wheel. In recent years, there has been a tendency to assemble the hub wheel, the constant velocity universal joint and the double row rolling bearing as a unit in order to reduce the weight and the size of the bearing apparatus for a driving wheel.
FIG. 12 is a prior art longitudinal cross-section view of a bearing apparatus for a driving wheel of vehicle where a hub wheel 50, a double row rolling bearing 60 and a constant velocity universal joint 70 are assembled as a unit. The hub wheel 50 is formed integrally with a wheel mounting flange 51 to mount a wheel (not shown) thereon and hub bolts 52 to secure the wheel are equidistantly arranged along the periphery of the flange 51.
The double row rolling bearing 60 comprises an outer member 61, an inner member 62 and double row rolling elements 63 and 63. The outer member 61 has a flange 64 integrally formed therewith to be mounted on a body of vehicle (not shown). The double row outer raceway surfaces 61a and 61a are formed on the inner surface thereof. In this case, the inner member 62 comprises the hub wheel 50 and an outer joint member 71. The inner member 62 is formed with double row inner raceway surfaces 50a and 71a arranged respectively opposite to the outer raceway surfaces 61a and 61a of the outer member 61. The inner raceway surface 50a is formed on the outer circumferential surface of the hub wheel 50 and the other raceway surface 71a is formed on the outer circumferential surface of the outer joint member 71 of the constant velocity universal joint 70. The double row rolling elements 63 and 63 are arranged between the outer raceway surfaces 61a and 61a and the inner raceway surfaces 50a and 71a and rotatably held therein by cages 65 and 65. Seals 66 and 67 are arranged at the ends of the double row rolling bearing in order to prevent leak of grease contained within the bearing and also to prevent incoming of rain water or dusts.
The constant velocity universal joint 70 comprises the outer joint member 71, a joint inner ring, a cage and a torque transmission balls (not shown). The outer joint member 71 has a cup shaped mouth portion 72, a shoulder 74 forming the bottom of the mouth portion 72, a shaft portion 73 axially extending from the shoulder 74, and axially extending curved track grooves 72a formed on the inner surface of the mouth portion 72.
The hub wheel 50 is formed with hardened irregular portion 53 on the inner circumferential surface thereof and has a cylindrical spigot portion 50b at its end. The hub wheel 50 and the outer joint member 71 are mutually connected by fitting the shaft portion 73 of the outer joint member 71 into the hub wheel 50 and then by radially outwardly expanding the fitting portion of the outer joint member 71 to cause plastic deformation of the outer joint member 71. The hardened irregular portion 53 to bite into the outer circumferential surface at the fitting portion of the outer joint member 71 (see Japanese Laid-Open Patent Publication No. 18605/2001).
Such a structure of connection via the plastic deformation can prevent loosening and abrasion of the fitted portion and thus can improve the durability and the driving stability. In addition, since the connection, via the plastic deformation, has both functions of torque transmission and connection of the hub wheel and the outer joint member, it is possible to eliminate securing means such as fastening nuts and to achieve reduction of the weight and size of the apparatus.
However, in such a bearing apparatus for a driving wheel, it suffers from repeating bending moment loads having its node at abutting portion between the hub wheel and the outer joint member when there would be caused an excessive bending moment during turning of the vehicle. In such a case, an excessive stress is caused in the spigot portion of the hub wheel and the shaft portion of the outer joint member, especially at the end of the shaft portion and thus the durability would be diminished. In addition, the repeated bending moment loads would cause deformations of the spigot portion and the shaft portion as well as abnormal abrasion at the abutted portion. Thus, this abrasion causes introduction of abraded debris into the bearing and thus fatigue life reduction of the apparatus.
In such a prior art bearing apparatus for a driving wheel of vehicle, it is possible to reduce the manufacturing cost of the hardened irregular portion and thus the cycle time of the step of radial expansion of the fitting portion of the outer joint member by reducing the length of the connecting portion via plastic deformation. However, it has been found that a desired lifetime cannot be achieved by the apparatus in which the length of the plastic deformed connecting portion is reduced to a length satisfying the allowable transmission torque according to a durability test carried out by the applicant of the present invention by applying the bending moment.